New modulation system for portable equipment



- June 4, 1946. H. N. KOZANOWSKI ,5 3

NEW MODULATION SYSTEM FORPORTABLE EQUIPMENT Filed March 27,1942

H. $52.3 m mm firwento: nowslii attorney Henry N: Kaza Patented June 4,1946 NEW MODULATION SYSTEM FOR PORTABLE EQUIPMENT Henry N. Kozanowski,Collingswood, N. J., asslgnor to Radio Corporation of tion of DelawareAmerica, a corpora- Application March 27, 1942, Serial No. 436,406

3 Claims. (Cl. 178-412) My invention relates to modulation and amplifiersystems and particularly to improved methods of and means fortransmitting picture signals or other high frequency signals.,

In portable television equipment one of the problems is to obtainsatisfactory synchronizing signal output from a transmitter with theminimum amount or weight of equipment. Another problem is to obtainsatisfactory operation from available vacuum tubes at very highfrequencies.

An object-of the present invention is to provide an improved method ofand means for modulating a radio transmitter by picture signals andsynchronizing pulses.

Another object of the invention is to provide arrangement of the poweramplifier of Fig. 1, and

Fig. 4 is a circuit'diagram of another embodiment of the invention.

In the several figures, corresponding parts-are indicated by the samereference characters.

In Fig. 1 the invention is applied to a television transmittercomprising a high frequency an improved amplifier for very highfrequency signals.

In one of the preferred embodiments of the invention, as applied to aportable television transmitter in which a high frequency carrier waveis amplitude modulated by both picture signals and horizontal andvertical synchronizing pulses, the picture signal modulation is obtainedby grid modulation while the synchronising pulse modulation is obtainedby plate modulation. In this way I avoid clipping the peaks ofsynchronizing pulses due to slight mistuning of the load, tank or gridcircuit or due to a change oscillator l0 and a power amplifier ll, eachhaving a, tuned-grid circuit and a tuned plate or anode circuit. Theoscillator anode circuit is coupled to the amplifier grid circuit asindicated by the curved arrow M.

The oscillator 10 includes a 'pair of vacuum I tubes l3 and Il, eachincluding for example 9.

directly heated cathode, a control grid and an anode. Heating current issupplied to each cathode through concentric lines l6 and I1. The sidesof 'the cathode filaments that are connected to the inner conductors ofthe concentric lines are also connected to the outer conductors throughcapacitors I8 and I9 to provide low impedance and symmetry of circuit atthe oscillator frequency. An adjustable shorting bar 2! is provided fortuning the line formed by the outer conductors of concentric lines l6and I I. A grid leak resistor 22 is connected between the control in thetotal signal level'that might result if grid moduiationwere employed forboth synchronizing pulses and picture signal. Also, by employing platemodulation for synchronizing it is possible to radiate synchronizingpulses containing more power with a given type or size of poweramplifier tube than is possible with grid modulation. a

In accordance with another feature of the invention I avoid thenecessity of cross-neutraliza tion or the like in the .power amplifierby utilizing triode amplifier tubes which have their control gridseffectively grounded at the carrier frequency, the cathodes of thetriodes being above ground potential at this frequency. Thus, thecontrol grids'function like screen grids to isolate the plate and gridcircuits whereby feedback is prevented.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing, in which Fig. 1 is acircuit diagram of a television transmitter embodying my invention,

Fig. 2 is a circuit diagram of another ment of the invention,

Fig.3 is a" perspective view of a mechanical embodiresistor 21,

grid of tubes I3 and H .and ground. The

anodes of tubes i3 and H are connected to the conductors 23 and 24 of aline tuned by a sh'orting bar 26. Anode voltage is supplied through aThe power amplifier ll includes vacuum tubes 3| and 32, each includingfor example a directly heated cathode, a control grid and a plate oranode. odes of tubes 3 l and 32 maybe supplied with heating currentthrough a pair of concentric lines 33 and 34 which are connected to 'atransformer 35. to capacitors l8 and 19 in the oscillator are provided.The outer conductors of lines 33 and 34 are provided with a groundedshorting bar 38 for tuning the cathode circuit.

The control grids'of tubes 3| and 32 are connected to ground at theoscillator or carrier frejquency through a series resonant circuit ll-l2tuned to said frequency. The cathode of. these tubes are above groundpotential at the carrier frequency since they are connected to thequarter-waveline 33-34 which is grounded at the opposite end.

The anode or plate circuit for tubes 3! and 32 comprises parallelconductors 33 and Hi and a shorting bar 48 which tunes the line to aquare As in the case of the oscillator. the cath- Capacitors 36 and 31corresponding the tube grids.

, ter wave length or less Plate voltage is supplied to the tubes 3| and32 through a constant impedance network of a well known type which maycomprise a resistor 41, inductance coils 48 and 49 shunted by resistors5| and 52', respectively. The network also includes shunt condensers 50and 55. Resistor 53 is a low impedance resistor for preventingsynchronizing pulses supplied through a conductor 63 and a cathodefollower tube 64 from asynchronizing pulse generator 66. Thepicture orvideo signa1 is impressed upon the grid circuit at the junction point ofa grid resistor 61 and a peaking coil 68 through which bias is appliedto This signal varies the control grids at video frequency with respectto cathode potential (also ground potential) although the grids remainat ground potential at the carrier frequency.

a plurality of power. amplifier tubes 3| and 32' put out much more powerthan they would for the same modulation power if they were platemodulated by thepicture signal having the wider frequency band.

As shown in Fig. 2, I may employ screen grid modulation in place ofplate modulation, the system otherwise being the same as in Fig.1. Itwill be noted that the synchronizing pulses are applied to both thescreen'grids and the anodes of the screen grid power amplifier tubes 3la and 321)..

The modulating pulses are applied to the anodes as well as to the screengrids to prevent the pulses from driving the screen grids to a higherpositive potential than the anodes.

In Fig. 3 I have-illustrated how, in one particular design, the controlgrids of the power amplifier tubes were brought to ground potential atthe carrier frequency. Instead of a single series res- The synchronizingpulses appearing across the cathode resistor H of amplifier tube 64 maybe applied to the junction 'point of resistor 53 and coil 49 wherebypositive pulses applied to thegrid of tube 64 drive the plates of tubes3| and 32 more positive to increase their output during the occurrenceof each synchronizing pulse.

The feature of amplitude modulating the grid with picture signals andthe plate with synchroupper knee of the curve obtained by plotting poweramplifier grid voltage against radio frequency output current. It maybe.noted that since the system shown in Fig. 1 employs A.-C.transmission (there being no D.-C. insertion) the "black" side of thepicture signals may sometimes be moved beyond the upper knee of thecurve also by an increase in the picture background component, that is;by the overall picture onant circuit 4I-42 tuned to the carrierfrequency, there are two such circuits in parallel (each inductance coil4i being in the form of a half loop).

The two resonant circuits ill-42 are connected between ground andopposite ends of a conducting plate 16 which functionsas a convenientbus bar to which the grid terminal leads or pins l1, l8, l9 and 8| areconnected. Plate terminal pins are shown at 82 and 83. It has been foundthat if only one resonant circuit is connected between the plate 16 andground, the neutralization or shielding may not be complete sincediflerent parts of the plate 16 tend to acquire different potentialsbecause of currents induced by the high frequency carrier wave.carrier-wave in the example described is of the order of 300 or 400megacycles per second.

becoming lighter in tone. Secondly, more synchroniz'lng power'can beobtained from given power amplifier tubes since the synchronizing pulsesmay drive theplate voltages momentarily beyond any plate voltage thatcan be applied to the tubes continuously'without damaging them. It mayaid in understanding my invention to point out why the advantages of thepresent invention cannot be' obtained by utilizing plate modulation forboth picture signals and synchro nizing pulses. It will be apparentforone thing that-more modulating power would be required .sinceplatemodulation inherently requires more power than grid modulation. Theother difflculty with the usebf plate modulation alone is that the poweramplifier output impedance (1. e., the

impedance of the constant impedance'network) I must be lowered as thefrequency band of the plate modulating signal is increased. In thesystem. shown in Fig. 1; the synchronizing signals have a frequency bandwidth of about 1 mega-. cycle whereas the picture signals have afrequency rier frequency.

closely enough in practice by giving the inductance coil U a very lowinductance and the con- It will be apparent that'the grids ofamplifiertubes 3| and 32 must be held at ground potential at the side bandfrequencies as well as at the car.-

This result is approximated denser 42 a comparatively high capacity (1.e. high capacity considering the frequency involved). For example, inFigs. 1 and 3 assuming the transmitter has a carrier-frequency of 350'megacycles per second the condenser 42 has a capacity of pedance of theresonant circuit 4 i--42 slightly off the resonant point is still verylow. In Fig. 4 I have shown how the control grids of tubes-3i and 32 maybe held at ground potential by means of a one-half wave length line86-81, instead of by a series resonant circuit like the circuit "-42 ofFig. 1. It will be understood that opposite ends of the one-half wavelength line are at the same potential at the carrier freuency and thatthe end remote from the control grids is at ground potential or itsequivalent since the amplifier circuit is of the balanced type. Itfollows that in this circuit-as in that of Fig. l, the plate and gridcircuits are shielded from each other at the carrier quency by theeffectively grounded control itispossibletousethetubcsataboutwperccntlThe frequency of the 5 higher frequency in practice than thatpermissible in a cross-neutralized circuit.

I claim as my invention:

1. In a television transmitter, means for producing a carrier wave, anamplifier connected to amplify said carrier wave, said amplifier havinga control grid and a plate, a source of picture signals andsynchronizing signals. said signals covering a wide frequency band and acomparatively narrow frequency band, respectively. means for amplitudegrid modulating said amplifier by said picture signals and means foramplitude plate modulating said amplifier by said synchronizing signals,and a constant impedance network through which a direct-currentoperating voltage is applied to said plate, said network having asubstantially constant impedance throughout said narrow frequency bandand only throughout said narrow frequency band whereby it presents amaximum impedance to said synchronizing signals.

' 2. In a television transmitter, means for producing a carrier wave, anamplifier connected to amplify said carrier wave, said amplifier havinga control grid, a screengrid, and a plate, a source of picture signalsand synchronizing signals, said signals covering a wide frequency bandand a comparatively narrow frequency band respectively, means forapplying said picture signals to the control grid ofsaid amplifier,means for applying said synchronizing signals to the screen grid andplate of said amplifier whereby the carrier wave a amplitude modulatedby both of said signals, and a constant impedance network through whicha direct-current operating voltage is applied to said plate, saidnetwork having a substantially constant impedance throughout said narrowfrequency band and'only throughout said narrow frequency band whereby itpresents a maximum impedance to said synchronizing signals.

3. In a television transmitter, meansfor producing a carrier wave, anamplifier tube for amplifying said carrier wave, said tube having acathode, a control electrode and at least one other electrode which isoperated at a positive potential with respect to the cathode, a sourceof picture signals and synchronizing signals, said signals covering awide frequencyband and a narrow frequency band, respectively, means forapplying said picture signals to said control grid to-modulate saidcarrier wave as to amplitude and means for applying said synchronizingsignals to said otherelectrode to further modulate said carrier wave asto amplitude, and a constant impedance network through which adirect-current operating voltage is applied to said other electrode,said network having a substantially constant impedance throughout saidnarrow frequency band and only throughout said narrow frequency bandwhereby it presents a maximum impedance to said synchronizing signals.

HENRY N. KOZANOWSKL

